The present invention is a development of the microcomputer described in our European patent specification No. 0113516. That specification describes the microcomputer with a memory and processor arranged to execute a plurality of concurrent processes and to provide synchronised communication between concurrent processes on the same microcomputer or interconnected microcomputers. However, the device described in that specification was arranged to transmit messages which were each of one word length. After execution of each message instruction one word is transmitted and subsequent message instructions are required in order to cause further words to be transmitted. In the case of a 16 bit machine each message would be of 16 data bits.